The body of fish is quite complex and multifunctional. The ability to stay under water with the performance of swimming manipulations and maintaining a stable position is determined by the special structure of the body. In addition to organs familiar even to humans, the body of many underwater inhabitants provides for critical parts that allow for buoyancy and stabilization. Essential in this context is the swim bladder, which is a continuation of the intestine. According to many scientists, this organ can be considered as a precursor to the human lungs. But in fish, it performs its primary tasks, which are not limited only to the function of a kind of balancer.
Formation of the swim bladder
The development of the bladder begins in the larva, from the foregut. Most freshwater fish retain this organ throughout their lives. At the time of release from the larva, the bubbles of the fry do not yet contain a gaseous composition. To fill it with air, the fish have to rise to the surface and independently capture the necessary mixture. At the stage of embryonic developmentThe swim bladder is formed as a dorsal outgrowth and is located under the spine. In the future, the channel that connects this part to the esophagus disappears. But this does not happen in all individuals. On the basis of the presence and absence of this channel, the fish are divided into closed- and open-bladed. In the first case, the air duct becomes overgrown, and gases are removed through the blood capillaries on the inner walls of the bladder. In open-bladder fish, this organ is connected to the intestines through an air duct, through which gases are excreted.
Gas bubble filling
Gas glands stabilize bladder pressure. In particular, they contribute to its increase, and if necessary, the red body is activated, formed by a dense capillary network. Since pressure equalization is slower in open-bladder fish than in closed-bladder species, they can quickly rise from the depths of the water. When catching individuals of the second type, fishermen sometimes observe how the swim bladder protrudes from the mouth. This is due to the fact that the container swells under conditions of rapid rise to the surface from the depth. Such fish, in particular, include zander, perch and stickleback. Some predators that live at the very bottom have a highly reduced bladder.
Hydrostatic function
The fish bladder is a multifunctional organ, but its main task is to stabilize the position in different conditions under water. This is a function of the hydrostaticcharacter, which, by the way, can be replaced by other parts of the body, which is confirmed by examples of fish that do not have such a bladder. One way or another, the main function helps the fish to stay at certain depths, where the weight of the water displaced by the body corresponds to the mass of the individual itself. In practice, the hydrostatic function can manifest itself as follows: at the moment of active immersion, the body contracts together with the bubble, and, on the contrary, straightens out during the ascent. During the dive, the mass of the displaced volume is reduced and becomes less than the weight of the fish. Therefore, the fish can go down without much difficulty. The lower the immersion, the higher the pressure force becomes and the more the body is compressed. The reverse processes occur at the moments of ascent - the gas expands, as a result of which the mass is lightened and the fish easily rises up.
Functions of the sense organs
Along with the hydrostatic function, this organ also acts as a kind of hearing aid. With its help, fish can perceive noise and vibration waves. But not all species have this ability - carps and catfish are included in the category with this ability. But sound perception is provided not by the swim bladder itself, but by the whole group of organs to which it is included. Special muscles, for example, can provoke vibrations of the walls of the bubble, which causes the sensation of vibrations. It is noteworthy that in some species that have such a bubble, hydrostatics are completely absent, but the ability to perceive sounds is preserved. This applies mainly to demersal fish, which spend most of their livesspend at the same level under water.
Protection functions
In moments of danger, minnows, for example, can release gas from the bubble and produce specific sounds that are distinguishable by their relatives. At the same time, one should not think that sound formation is of a primitive nature and cannot be perceived by other inhabitants of the underwater world. Croakers are well known to fishermen for their rumbling and grunting sounds. Moreover, the swim bladder, which trigle fish have, literally terrified the crews of American submarines during the war - the sounds made were so expressive. Usually, such manifestations take place at moments of nervous overexertion of the fish. If in the case of the hydrostatic function, the operation of the bubble occurs under the influence of external pressure, then sound formation occurs as a special protective signal formed exclusively by fish.
What fish don't have a swim bladder?
Deprived of this organ are sailing fish, as well as species that lead a demersal lifestyle. Almost all deep-sea individuals also do without a swim bladder. This is exactly the case when buoyancy can be provided in alternative ways - in particular, thanks to fat accumulations and their ability not to compress. The low density of the body in some fish also contributes to maintaining the stability of the position. But there is another principle of maintaining the hydrostatic function. For example, a shark does not have a swim bladder, so itforced to maintain a sufficient depth of immersion through active manipulation of the body and fins.
Conclusion
It is not without reason that many scientists draw parallels between the human respiratory organs and the fish bladder. These parts of the body are united by an evolutionary relationship, in the context of which it is worth considering the modern structure of fish. The fact that not all fish species have a swim bladder causes its inconsistency. This does not mean at all that this organ is unnecessary, but the processes of its atrophy and reduction indicate the possibility of doing without this part. In some cases, fish use internal fat and lower body density for the same hydrostatic function, while in others they use fins.
